FIG. 4 is a schematic view showing a basic concept of an exemplary inductively-coupled plasma mass spectrometer (hereinafter, also referred to simply as instrument) 11. The instrument 11 has a plasma torch 20 for generating plasma 22, an interface section 30 placed at a position facing the plasma 22, an ion lens section 50 placed behind the interface section 30, an ion guide section 70 placed behind the ion lens section 50, and an ion separation section 80 placed behind the ion guide section 70.
The plasma torch 20 has a coil 21 for generating a high frequency electromagnetic field near its tip, and is placed under atmospheric pressure. The coil 21 is connected to an RF power source (not illustrated). In the plasma torch 20, the high frequency electromagnetic field generated by the coil 21 produces high frequency inductively-coupled plasma 22 under atmospheric pressure. In the plasma torch 20, an atomized sample (not illustrated) is introduced into the plasma 22 from the front of the plasma torch 20. The introduced sample (not illustrated) is vaporized and decomposed by the action of the plasma 22; and in cases of large majority of elements, they are finally converted into ions. The ionized sample (not illustrated) is contained in the plasma 22.
Ions in the plasma 22 pass through a sampling cone 31 and a skimmer cone 33 of the interface section 30, and only positive ions are extracted in the form of an ion beam by a first electrode 53 and a second electrode 54 forming an extraction electrode section of the ion lens section 50. Then, the ion beam is guided into a collision/reaction cell 71 of the ion guide section 70. The ion beam guided into the collision/reaction cell 71 is induced to a subsequent stage along a track determined by an electric field generated by a multipole electrode (e.g., an octapole structure) 73. Further, a collision/reaction gas may be introduced from a feeding port 72 into the collision/reaction cell 71, and polyatomic ions or interference ions that cause interferences in mass spectra are removed from the ion beam. During operation of the instrument 11, the interior of the interface section 30 is sucked by a rotary pump RP, the interiors of the ion lens section 50 and the ion guide section 70 are sucked by a turbo molecular pump (TMP 1), and the interior of the ion separation section 80 described below is sucked by a turbo molecular pump (TMP 2).
The ion beam having passed through the collision/reaction cell 71 is introduced into a mass analyzer (generally, a quadrupole mass analyzer or a quadrupole mass filter) 81 of the ion separation section 80. Ions in the ion beam are separated based on the mass-to-charge ratio, and separated ions are guided and detected in an ion detector 82. Detection signals are computed by a signal processing section 90 and analytical values of an element to be measured in a sample are obtained.
The ion detector 82 may generally be a secondary electron multiplier capable of detecting an imperceptible ion stream with high precision. For example, a secondary electron multiplier is disclosed in Patent Document 1 (Japanese Patent Laid-Open Publication No. H5-325,888). A secondary electron multiplier utilizes a property, which emits secondary electrons by collision of ions with a metal surface or a surface of specially-treated ceramic. The emitted secondary electrons are accelerated by an electric field, further subjected to repeated collisions, and thereby they are exponentially amplified. In general, secondary electrons can be amplified in the order of about 103 to about 108. As a material for emission of secondary electrons, a metal such as Al or Cu—Be alloy with an oxidized surface, ceramic or the like may be used. FIG. 5 schematically shows a construction of an exemplary secondary electron multiplier 10 of conventional art. The secondary electron multiplier 10 shown in FIG. 5 is generally called as a secondary electron multiplier of multiple-stage dynode type, having a structure where a plurality of electrodes called as dynodes are aligned to face one another. In FIG. 5, an exemplary construction with 10-stage dynode is shown, but a construction with about 20 or more stages may be generally used. A first dynode is called as a conversion dynode to convert an ion to an electron. Therefore, when a secondary electron multiplier 10 is used as an ion detector 82 in a mass spectrometer shown in FIG. 4, ions having passed through the mass analyzer 81 collide with a surface of this conversion dynode dy1. Second and subsequent dynodes mainly work to amplify secondary electrons. Finally, multiplied electrons are detected at a final stage (e.g., anode in FIG. 5) and output to the signal processing section 90.
At the signal processing section 90, a signal can be measured by two kinds of method. One is a method for obtaining the number of ions having reached the ion detector by counting current pulses of amplified electrons (pulse counting method); and the other is a method for obtaining a value proportional to the number of ions having reached to the ion detector by measuring a current of amplified electrons as a DC value (current measurement method).
A high negative voltage −V is applied to the conversion dynode dy1 by a power source 85. In a standard secondary electron multiplier, about −1500 V to about −3500 V may be applied to the conversion dynode dy1. Therefore, the power source 85 has a variable output voltage, and a control signal from, for example, a controller (not illustrated) enables the control of the output voltage. As shown in FIG. 5, dynodes dy1 to dy10 and an anode are connected in series via respective resistors R1 to R11. To each of a dynode dy2 at a second stage to dy10 and the anode, a voltage sequentially divided by each of resistors R1 to R11 is applied. In a secondary electron multiplier of high energy dynode type (not illustrated), a voltage of about −10 kV may be applied to a conversion dynode at a first stage.
It is probable that the yield of ion/electron conversion at the first-stage dynode dy1 exerts a critical influence on the efficiency of ion detection. It is generally known that the yield of ion/electron conversion statistically follows Poisson distribution. Assuming that the mean yield is 1, about 37% of ions incident in a secondary electron multiplier do not emit even one electron. As a result, no output signal is output from the secondary electron multiplier. However, if the mean yield is increased to 3, the amount of ions that emit no electron decreases to about 5% of the entire ions, resulting in increased efficiency of ion detection. The improvement of ion detection efficiency by this increase of ion/electron conversion yield provides an increase in the measurement sensitivity in both of the pulse counting method and the current measurement method. Further, the ion/electron conversion yield correlates with a kinetic energy of incident ions, that is a first dynode voltage, and a higher ion kinetic energy can provide a higher conversion yield.
FIG. 7 shows the relationship between the voltage applied to a first dynode dy1 and the ion detection sensitivity. A graph of FIG. 7 shows measurement results of lithium (7u), yttrium (89u) and thallium (205u) by changing voltages to be applied to the first dynode dy1 under the condition where the secondary electron multiplier has a constant amplification gain in an ICP mass spectrometer 7700× manufactured by Agilent Technologies, Inc. In the case of lithium having a low mass number, there is almost no increase in the sensitivity. It is observed that an element having a larger mass number has a larger increase in the sensitivity. This is because the difference in the ion mass number leads to a difference in the ion/electron conversion efficiency characteristic on the voltage applied to the first dynode dy1. In the case of an element having a low mass number, it is inferred that the ion/electron conversion efficiency is not so much increased even by decreasing a voltage to the first dynode dy1.
It is commonly known that a secondary electron multiplier gradually decays with the use thereof, and its amplification gain also gradually decreases. Therefore, the decreased amplification gain can be recovered by further reducing a negative voltage applied to the first dynode dy1 (increasing an absolute value of the voltage). However, an excessive increase of the voltage between dynodes saturates the emission of secondary electrons from the dynodes, so there is a limit on the recovery of the amplification gain. FIG. 6 shows a typical example indicating the relationship between the electron energy and the secondary electron emission.
For example, FIG. 8 is a drawing wherein voltages of each dynode are plotted when a usable period of a standard secondary electron multiplier is divided, for example, into three stages, “initial,” “middle” and “final.” It shows that when the amplification gain is kept constant, a negative voltage applied to the first dynode dy1 is decreased in a step-by-step manner along with the deterioration of the secondary electron multiplier. FIG. 8 is prepared based on the premise that the secondary electron multiplier has 10 stages of dynodes, and up to −2500 V can be applied to the first dynode dy1. In order to increase the ion detection efficiency in such electron multiplier, a case where a negative voltage to be applied to the first dynode dy1 is lower than an ordinary voltage (larger in the absolute value of voltage) is premised. FIG. 9 shows a drawing wherein voltages of each dynode in such case are plotted. As shown in FIG. 9, a voltage applied to the first stage at “initial” is reduced from an ordinary value, about −1500 V (FIG. 8) to about −2000 V; at “middle,” from about −2000 V (FIG. 8) to about −2250 V (at “final,” remaining at about −2500 V). In comparison with the case of FIG. 8, this case cannot provide a sufficient voltage margin required for recovering a reduction of amplification gain caused by the deterioration of a secondary electron multiplier by reducing a negative voltage applied to the first dynode dy1. Therefore, a secondary electron multiplier of this case shortens its usable period (lifetime). Further, in a secondary electron multiplier of high energy dynode type (not illustrated), a high negative voltage (e.g., about −10 kV) is applied to a conversion dynode dy1, so the ion/electron conversion yield at the first dynode dy1 is higher in comparison with a standard secondary electron multiplier. However, since the voltage to a second dynode dy2, which affects the amplification gain of a secondary electron multiplier, is usually about −2000 V, an incident energy of electrons to the dynode dy2 is too high and consequently, it is considered highly probable that a considerable amount of current pulse signal is lost due to this dynode dy2.
Therefore, there is a need for improving the ion detection efficiency of a secondary electron multiplier without a large reduction of a usable period of thereof, and consequently increasing the sensitivity of a mass spectrometer.